I think I see why this might be confusing.
A duplex is the building itself. It's a home that is divided into two apartment units. For singular you might say, "I rent an apartment in a duplex in this area", or "I have a duplex apartment in this area". If I own multiple buildings that are divided into duplexes I would say, "I have duplexes in this area", or "I have duplex apartments in this area."
I hope this helps and isn't more confusing!